1. Field of the Invention
The present invention generally relates to antenna arrays and, more particularly, to the calibration of antenna arrays.
2. Background of the Invention
Modern RF communication systems often include antenna arrays, which generally improve their wireless communication performance and capacity. In an antenna array, a plurality of antennas are spatially arranged and their respective transceivers are electrically connected via a feed network so as to cooperatively transmit and/or receive RF signals using beamforming or other multi-antenna transmission and reception techniques. Beamforming enables an antenna array to achieve high gain and controlled beamwidth in desired directions, while minimizing transmission and reception of RF signals in other directions.
To implement beamforming in transmit mode, the same signal may be simultaneously applied to the transmit apparatus chain for each of the antennas of the antenna array, thereby creating an individual instance of the signal for each of the respective antennas. One or more of the signal instances may be processed, for example to adjust their respective phases and/or amplitudes, in order to achieve a desired radiation pattern. When using an antenna array in receive mode, the same RF signal can be detected by each of the antennas and propagated to respective receive apparatus chains for processing and combining. For example, receive beamforming can be implemented by adjusting the phase and/or amplitude of one or more of these signal instances to result in a desired reception pattern when the individual signal instances are combined during receive processing.
The accuracy of an antenna array's beamforming characteristics typically depends on the accuracy of the knowledge of the characteristics of the antenna array's transmit/receive apparatus chains, as well as environmental conditions through which the RF signals are wirelessly transmitted. Thus, to insure proper operation, antenna arrays oftentimes are calibrated using a dedicated calibration transceiver to generate calibration coefficients for each antenna to compensate for variations in signal propagation mediums, including the transmit/receive apparatus chains and the wireless medium(s). The respective calibration coefficients then may be applied to the RF signal instances that are sent to, or received from, corresponding antennas. Typically, the calibration coefficients are applied during signal baseband processing.